Safety circuits with safety switch means are used for switching safety-relevant actuators, as for example a drive motor, which is switched via a motor contactor. For monitoring the operating status of the actuator safety circuits may use feedback signals of the actuator, for example via an auxiliary contact of the contactor. In case of a failure, if the current switching signal does not coincide with the feedback signal, such a safety circuit causes the actuator to be disconnected from the grid by ease of the safety switch means. Depending on the required security level safety circuits can also be configured redundantly.
The operability of the safety switching means has thereby to be ensured at any time. Preferably semiconductor switches are used as safety switch means, since then the use of metal contacts can be avoided, which may wear out, oxidize, or even stick together. For testing the function capability of the safety switch means, the switching status is changed in regular intervals, in order to check the proper switching of the safety switch means. After the proper functioning of the safety switch means is assessed, the change of the switching status of the safety switch means is reversed. The time interval between the two switching operations should be relatively short. In any case, the time interval should be short enough so that due to the inertia of the actuator, the operating status of the actuator is not changed in order to prevent accidental switch off of the actuator.
In the prior art, the monitoring of the function of the safety switch means is tested by ease of a programmable logic. For this purpose, a test signal is generated by the programmable logic that changes the switching status of the safety switch means. After the successful detection of the switching via a monitoring input, the programmable logic switches the safety switch means on again.
As described for example in DE 195 08 841 A1, a computing unit can change the switching status of the safety switch means for the length of a test pulse. The length of the test pulse ideally is adapted by the inertia of the actuator to be switched, so that the operating status of the actuator during the test will not be changed. The successful change of the switching status is displayed during the test at an input of the computing unit.
In EP 2 149 826 B1 the function monitoring is described by ease of a programmable logic, wherein the programmable logic enables a simple adaptation of the function monitoring of the safety switch means to be tested and the actuator used.
However, such function monitoring have the disadvantage that the programmable logics used, such as micro controllers or computational units must be sufficiently fast to recognize the successful change of the switching status at the corresponding input of the programmable logic, since the associated signal is there only present for a short duration of time. Therefore, only very high clocked programmable logics can be used for the function monitoring, as only these programmable logics meet the speed requirements. However, such programmable logics are comparatively expensive. Furthermore, such monitoring function have the disadvantage that the switching time depends on the speed and software architecture of the programmable logic and the duration of the test signal respectively the test pulse which may need to be adjusted manually to the inertia of the component to be switched.
The object of the present invention is therefore to provide an apparatus for monitoring the function of a safety switch means that is inexpensive to manufacture, is independent of the speed of the programmable logic and which independent of the inertia of the component to be switched provides a reliable result.